1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of performing a random access procedure in a wireless communication system.
2. Related Art
Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference with a low complexity is taken into consideration as one of next generation (after the third generation) systems. In the OFDM, a data stream is transmitted by being carried on a plurality of subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal subcarrier experiences independent frequency selective fading. Inter-symbol interference can be removed by using a cyclic prefix (CP).
Orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and thus the respective frequency resources are independently provided to the plurality of users.
A user equipment (UE) generally performs a random access procedure to access to a network. The random access procedure is performed to adjust uplink synchronization or to request an uplink radio resource assignment. For one example, the UE may perform the random access procedure to acquire uplink synchronization after adjusting downlink synchronization when power is initially turned on. For another example, in a state where a radio resource control (RRC) connection is not established, the UE may perform the random access procedure so that uplink radio resources are allocated for uplink transmission. For another example, the UE may perform the random access procedure so that initial access to a target base station (BS) is achieved in a handover procedure.
Since the random access procedure is an initialization procedure for uplink transmission or for network access, delay or failure in the random access procedure causes a service delay. Accordingly, there is a need for a method capable of performing the random access procedure in a more rapid and reliable manner.